The present invention relates to a light interference film which selectively transmits or reflects light of a desirable wavelength range, and especially transmits yellow light, and a lamp using the light interference film.
In a recently proposed halogen lamp having a glass bulb, an infrared ray reflecting film through which visible light passes is formed on the glass bulb within which a filament is provided. Amongst the light emitted by the filament, infrared rays are reflected from the infrared ray reflecting film and returned to the filament. Thus, the infrared rays reflected from the reflecting film heats the filament and the emitting efficiency is improved. At the same time, the amount of infrared rays transmitted outside the lamp is reduced.
Such an infrared ray reflecting film is formed by alternately stacking high-refractive index layers of titanium oxide (TiO.sub.2) or the like and low-refractive index layers of silicon oxide (SiO.sub.2) or the like. The infrared ray reflecting film consists of more than 5-7 layers.
This type of film is called a light interference film. The application of the light interference film in order to obtain desirable wavelength light such as yellow light has been studied.
In general, the light interference film comprises odd layers whose optical thickness nd (n=refractive index, d=thickness)=.lambda..sub.0, and odd layers whose optical thickness nd=(1/4).lambda..sub.0 stacked alternately. This light interference film is called (1/4).lambda. film, and .lambda..sub.0 corresponds to a maximum reflectance.
Otherwise, the light interference film is formed by stacking another low-refractive layers (nd=(1/8).lambda..sub.0) having a half thickness thereof so that the transmittance may be even. Furthermore, layers whose nd=(1/2).lambda..sub.0 or layers whose nd=(1/8).lambda..sub.0 are appropriately inserted to the stacked layers, so that the transmittance and the reflectance of the light interference film may be adjusted.
As to the lamp having the light interference film, it has been studied that the wavelength range less than 500 nm is reflected from the film and the wavelength range more than 500 nm is transmitted through the film in order to obtain yellow light. However, it is actually difficult to form such a light interference film.
Furthermore, in general, the light interference film has an incident angle dependency, accordingly as the incident angle with respect to the light interference film leaves from 0.degree., a transmission characteristic is shifted toward a short wavelength side and the reflectance characteristic is reduced.